This disclosure relates generally to methods and apparatus for generating pulses in a fluid column, as may be used for telemetry between a surface location and downhole instrumentation within a subterranean well.
Drilling fluid circulated down a drill string to lubricate the drill bit and remove cuttings is typically broadly referred to as drilling “mud.” The generation of pulses in a drilling fluid column to communicate information to the surface is generally termed “mud pulse telemetry.” Numerous mud pulse telemetry systems have been developed, using various forms of valve mechanisms, typically disposed in the drill string, to produce fluid pulses. Some mechanisms provide a bypass for the circulating fluid from the interior of the drill string to the wellbore annulus to create a controlled, momentary pressure drop or “negative pulse.” Other mechanisms create a controlled restriction in the fluid path, causing a controlled, momentary pressure increase or “positive pulse.” Such mechanism may utilize, for example, a “poppet” valve with a valve member that linearly reciprocates to open and close a fluid passageway.
An alternative approach to linear reciprocation is provided by the use of a rotary valve that can generate a continuously variable carrier wave onto which a signal is imparted by modulation. Apparatus implementing this approach are often referred to as “mud sirens.” A rotary valve may include, for example, a rotor that rotates, relative to a stator, around an axis parallel to the fluid flow (rotating either reciprocally or continuously in the same direction) to periodically open and close one or more fluid passageways. Each of these systems offers various features and characteristics.